The present invention relates to FM demodulators and more particularly to a quadrature type FM demodulator which may be utilized with a plurality of different bandpass filters selected in accordance with the bandpass of a receiver with which the demodulator is to be employed.
Usually, quadrature FM demodulators employ a limiter to provide one signal to a double-balanced demodulator mixer and provide a quadrature signal by loosely coupling to a single tuned circuit also driven by the limiter. Such an arrangement is restricted to relatively narrow frequency bands in use, for instance, in the FM sound carrier of a TV signal. The system is restricted to use with a single bandwidth and cannot be accommodated to variable bandwidth systems.
Another method of obtaining the quadrature signal is to use a delay line, but again, the delay line is operative only at a single frequency.
It is an object of the present invention to provide a quadrature type FM detector for use with various bandwidth circuits and specifically, to provide a quadrature type FM demodulator having circuitry which is common to all bandwidths at which the demodulator can operate except the specific bandwidth determining circuits, such circuits comprising a plurality of filters individually switched into the circuit with each providing an exact 90.degree. phase shift therethrough at the FM carrier frequency and providing a linear frequency to phase shift function through each of the filters whereby there is produced at the output of the demodulator, a d.c. signal which varies as a function of the FM modulation, and further, to provide a separate d.c. gain control for the double-balanced mixer each for insertion in the circuit in conjunction with a different filter whereby to provide the same zero signal d.c. level on the output of the mixer regardless of the filter employed for each bandwidth.
More specifically, the present invention provides a limiter-driver for providing FM signals, with the AM substantially removed, to a signal splitter wherein one output of the splitter is provided as an input to a double-balanced mixer. The second signal from the splitter is provided to a bank of bandpass filters each having a different bandpass characteristic. Each filter has a bandpass characteristic which is compatible with a selectable bandpass characteristic of the receiver or other equipment with which the FM demodulator is to be employed. At the carrier frequency of the FM carrier, the phase shift through each filter is 90.degree., as previously indicated, and as the frequency varies with signal, the phase shift through the filter varies linearly above and below the 90.degree. shift. The output signal from the selected filter is applied to the double-balanced mixer which produces a determinable d.c. level at its output as a function of the delay of the signal through the selected bandpass filter. Thus, as the frequency of the signal rises above carrier frequency, the phase shift through the filter increases and as the frequency falls below the carrier frequency, the phase shift decreases. In consequence, as frequency rises, the d.c. level at the output of the double-balanced mixer rises; as the frequency falls, the output of the double-balanced mixer falls. Since the filters are such that the shift at the carrier frequency is 90.degree. through each filter and the frequency-phase relationship is linear for all four filters, the FM signal is converted to a phase modulated signal that is readily detected by the double-balanced mixer. Thus, a single driver circuit for the filters and the demodulator and output circuits may be common for all filters having the characteristics as defined above.
Characteristics of the filters other than the frequency-phase shift characteristics are somewhat different and the d.c. level of the output signal from the modulator is not the same for each filter unless steps are taken to change the gain of the mixer for each filter selected. In accordance with a further feature of the invention, the resistance value in bias circuit of the mixer is changed in accordance with each filter characteristic so that the d.c. gain level of the mixer for each filter is normalized for the respective bandwidth deviations. The output level from the demodulator is not balanced relative to ground, but rides at a potential above ground which may be somewhat substantial. In the specific circuits employed in the present invention, the offset above ground is about 9 volts but varies somewhat with the particular filter employed. In accordance with a further feature of the present invention, a variable offset control is provided to the output from one leg of the balanced output from the mixer to permit precise individual control of offset correction for each filter.